Inhibition of free radical-mediated oxidation is one of the most important reactions in organic substrates and is commonly used in rubbers, polymers and lubrication oils; namely, since these chemical products may undergo oxidative damage by the autoxidation process. Hydrocarbon oxidation is a three step process which comprises: initiation, propagation and termination. Oxidative degradation and the reaction mechanisms are dependent upon the specific hydrocarbons, temperatures, operating conditions, catalysts such as metals, etc., for which more details can be found in Chapter 4 of Mortier R. M. et al., 1992, “Chemistry and Technology of Lubricants Initiation”, VCH Publishers, Inc.; which is incorporated herein by reference in its entirety. Initiation involves the reaction of oxygen or nitrogen oxides (NOx) on a hydrocarbon molecule. Typically, initiation starts by the abstraction of hydrocarbon proton. This may result in the formation of hydrogen peroxide (HOOH) and radicals such as alkyl radicals (R•) and peroxy radicals (ROO•). During the propagation stage, hydroperoxides may decompose, either on their own or in the presence of catalysts such as metal ions, to alkoxy radicals (RO•) and peroxy radicals. These radicals can react with the hydrocarbons to form a variety of additional radicals and reactive oxygen containing compounds such as alcohols, aldehydes, ketones and carboxylic acids; which again can further polymerize or continue chain propagation. Termination results from the self termination of radicals or by reacting with oxidation inhibitors.
The uncatalyzed oxidation of hydrocarbons at temperatures of up to about 120° C. primarily leads to alkyl-hydroperoxides, dialkylperoxides, alcohols, ketones; as well as the products which result from cleavage of dihydroperoxides such as diketones, keto-aldehydes hydroxyketones and so forth. At higher temperatures (above 120° C.) the reaction rates are increased and cleavage of the hydroperoxides plays a more important role. Further polycondesation and polymerization reaction of these high molecular weight intermediates results in products which are no longer soluble in the hydrocarbon and form varnish like deposits and sludge.
Since autoxidation is a free-radical chain reaction, it therefore, can be inhibited at the initiation and/or propagation steps. Typical oxidation inhibition by diarylamines, such as dialkyldiphenylamine and N-phenyl-α-napthylamine, also involves radical scavenging. The transfer of hydrogen from the NH group of the amine to the peroxide radicals results in the formation of a diarylamino radical which is resonance stabilized, thus prevents new chains from forming. A secondary peroxy radical or hydroperoxide can react with the diarylamino radical to form the nitroxy radical, which is also a very potent inhibitor.